boot: improve support for qemu

systemd-boot expects being loaded from ESP and is quite unhappy in case
the loaded image device path is something else.  When running on qemu
this can easily happen though.  Case one is direct kernel boot, i.e.
loading via 'qemu -kernel systemd-bootx64.efi'.  Case two is sd-boot
being added to the ovmf firmware image and being loaded from there.

This patch detects both cases and goes inspect all file systems known to
the firmware, trying to find the ESP.  When present the
VMMBootOrderNNNN variables are used to inspect the file systems in the
given order.

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